PTL 1 discloses a mechanism including a guide attached to a lower part of a distal end portion side surface of a base end boom, and a guide roller attached to a substantially central portion of a jib. Further, PTL 1 discloses a mechanism including a boom side pin socket attached to a lower part of a side surface of the central portion of the base end boom, and a jib side fixing pin that is attached to a distal end portion of the jib.
In order to perform jib overhanging work, a jib base end portion and a boom distal end portion are connected first. Subsequently, a boom is raised. When the boom is slightly extended next, the jib side fixing pin is detached from the boom side pin socket. When the boom is further extended, a guide roller rolls along a sloping surface of a guide, and the jib slowly separates from a bottom surface of the base end boom. When the guide roller is completely detached from the guide, the jib is brought into a state where the jib is suspended from the boom distal end portion. Finally, tension is generated in a tension rod, and thereby the jib is overhung.
Jib storage work is performed in the reverse order of the jib overhanging work. When the boom is contracted in a state where the jib is suspended from the boom distal end portion, the guide roller rolls along the guide, and the jib is pulled toward the bottom surface of the base end boom. When the boom is further contracted, the jib side fixing pin is inserted into the boom side pin socket. Thereby, the jib can be fixed to the boom.
There is a need for applying the mechanism described in PTL 1 also to large cranes. The jib loaded on a large crane is long and has a heavy weight, and therefore has large deflection. In addition, each part has large play in a large crane. Consequently, even when the jib is pulled toward the bottom surface of the base end boom by the guide roller and the guide in jib storing work, the position of the jib side fixing pin is deviated by deflection and play. Hence, there is a problem that the jib side fixing pin cannot be inserted into the boom side pin socket, and the jib cannot be stored.
In recent years, a so-called round boom is often used in which the sectional shape of a boom bottom portion is formed in a circular arc shape for the purpose of reducing the weight and increasing rigidity (for example, PTL 2). In a round boom, which is weaker to constrain rotation than the booms having quadrangular and hexagonal sectional shapes, the distal end boom easily rotates with respect to the base end boom. The position of the jib side fixing pin is also deviated by the rotation, so that the jib side fixing pin cannot be inserted into the boom side pin socket.